1. Field of the Invention
This invention relates to an electrically driven type injection apparatus, and more particularly to an electrically driven type injection apparatus in which the rotational movement and rectilinear movement of a screw for kneading and injecting a material to be molded are effected by an electric motor.
2. Related Background Art
In an injection molding method wherein a material to be molded such as plastic is injected into a molding metal mold to thereby manufacture a molded article, use has heretofore been made chiefly of hydraulic type and electrically driven type injection apparatuses.
Here, a hydraulic type injection apparatus according to the prior art will first be described with reference to FIG. 9 of the accompanying drawings.
This hydraulic type injection apparatus is comprised chiefly of a hopper 71, a screw 73 for mulling and measuring resin amount charged from the hopper in a cylinder 72, a motor 51 for rotatively driving the screw, and a direct-acting type cylinder 52 for injecting the kneaded resin, and the motor 51 and the direct-acting type cylinder 52 have usually adopted the hydraulic driving type in which a great output can be easily obtained.
An electrically driven type injection apparatus disclosed in Japanese Patent Publication No. 61-57168 will now be described with reference to FIG. 10 of the accompanying drawings.
In FIG. 10, the reference numeral 53 designates a screw contained in a heating cylinder 66. This screw is fixed to a driven gear 54 for rotating the screw which is disposed rearwardly. The reference numeral 56 denotes a support member slidably guided by a guide bar 57. This support member rotatably supports the driven gear 54 for rotating the screw and has secured thereto a ball nut 55 to which is fitted a ball bearing screw 58 whose fore end bears against the shaft of the gear 54. A driven gear 59 for propulsion is secured to the ball bearing screw 58. The driven gear 54 for rotating the screw and the driven gear 59 for propulsion are disposed above the rotary shaft of a motor 62, and are connected to driving gears 63 and 64, respectively, which are connected together by clutches 60 and 61.
In this apparatus, there is also provided a back pressure brake unit 65 behind the driven gear 59 for propulsion so that the retracting operation of the screw 53 may be effected from rearward of this gear 59. Thus, when the screw 53 is retracted by the kneading and measurement of resin charged from a hopper 74 into a heating cylinder 66, the ball nut 55 retracts through the gear 54 and the support member 56, and the ball bearing screw 58 is rotated and in accordance therewith, the gear 59 is rotated. When the end surface of this gear 59 which is being rotated is pressed by the back pressure brake unit 65, the rotation of the gear 59 can be braked by a slip torque created between the two and thus, the back pressure against the screw 53 may be provided.
However, the above-described injection apparatuses according to the prior art have suffered from problems as mentioned below.
In the hydraulic type injection apparatus as shown in FIG. 9,
(1) a hydraulic pump and surrounding devices such as piping facilities are required and therefore, a wide installation space for the injection molding apparatus is required, and
(2) because of oil mist or the like produced from a hydraulic driving instrument, it is impossible to use the injection molding apparatus under a clean environment.
On the other hand, in the electrically driven type injection apparatus as shown in FIG. 10, the problems peculiar to the hydraulic type injection apparatus are eliminated, but
(1) to impart back pressure, it is necessary to control this back pressure by the sum of a force for converting the rectilinear movement of the ball nut, etc. during the retraction of the screw into the rotational movement of the ball bearing screw, etc. and a force produced from slip torque by a brake pressed against the end surface of a gear or the like and therefore, there are required many parameters for the back pressure (such as the rotational resistance of the ball bearing screw or the like, the frictional force of a brake plate and the produced output of the brake) and the setting of conditions becomes complicated, and
(2) because of a construction in which the rotatively driven gear and the propulsion-driven ball nut are supported by one and the same support member and are moved with the screw at a time, a guide for stopping the rotation of the support member becomes necessary and a wide installation space around the driving system is required, and the construction becomes complicated.